As a rule, teenage boys and young men constitute a healthy chunk of the audience for comic book movies, and going with the 17-and-older approach probably won't work for the high stakes bingo of Batman and Superman movies, not that there could ever realistically be an R-rated Superman movie.
But that dream of Zack Snyder filming the fourth new Batman movie and tackling Frank Miller's Dark Knight Returns graphic novel saga probably will never come to fruition.
Again, from the perspective of spending hundreds of millions of dollars and hoping for hundreds of millions more, PG-13 is definitely the way to go. I'd be shocked if the studio let its first movie of the summer come out with the R rating. The simplest explanation will be, "Wait for the DVD."